In recent years, studies on practical application of light weight Al group plates to external panels of automobile body as a substitute for the existing cold-drawn steel panels have been aggressively promoted to reduce the weight of automobile bodies.
Chemical conversion treatment with a phosphate is usually applied before painting to the cold-drawn steel plates, which are widely used for automobile bodies, to give corrosion resistance. Aluminum group plates also require similar pretreatment. Direct phosphate treatment on an Al group plate, however, results in significant degradation of treatment efficiency because of the inhibition effect of the dense oxide coating which is formed on the surface of the plate causing a degradation of the improvement effect against filiform rusting. Furthermore, since Al ions dissolve into the phosphate treatment solution to degrade the treatment efficiency, the smooth formation of overall phosphate coating is inhibited in the case of simultaneous treatment of steel plate parts with Al plates.
As countermeasures to such problems, Al group plates treated to form a metallic plating layer, such as Zn plating, thereon prior to the phosphate treatment have been proposed: JP-A-61-157693; JP-A-63-153262; JP-A-63-166964 (the term "JP-A-" refers "unexamined Japanese patent publication").
In the prior art, the main goals of forming a preliminary metallic layer, such as a Zn plating layer, on an Al group plate are to enable the simultaneous phosphate treatment of Al group plates, steel plates, and surface-treated steel plates, and to improve the chemical conversion treatment efficiency by preventing possible dissolution of Al ions and accumulation thereof.
An essential factor in the prior art is the presence of a metallic layer, such as a Zn plating layer, as an intermediate layer between an Al group plate and phosphate coating after the phosphate treatment. To realize such a sandwich configuration, a metallic layer, such as a Zn plating layer, must be formed to a thickness sufficient enough to prevent dissolution during the chemical conversion treatment. This thickness should correspond to a minimum of coverage 0.4 g/m.sup.2.
In concrete terms, JP-A-61-157693 describes the use of an Al plate having a Zn plating layer of 1 g/m.sup.2 or higher, JP-A-63-153262 specifies an Al plate with a Zn plating layer of 0.4 g/m.sup.2 or higher formed thereon, and JP-A-63-166964 describes an Al plate plated with a coating weight of 0.4.about.5.0 g/m.sup.2 of Zn.
According to studies performed by the inventors of this invention, however, the application of an Al group plate having a Zn coverage of 0.4 g/m.sup.2 or higher to automobile body materials tends to result in a residual Zn layer on the Al plate after chemical conversion treatment. This residual Zn layer has been found to cause blistering during corrosion testing carried after painting.